The sun shone then too. When Labour gathered in Brighton in 1997, Tony Blair's talk of a "new dawn" fresh in its collective ears, the sky remained blue and cloudless, as if smiling on the journey about to begin. Photographers snapped the young prime minister, eyes closed, his face turned to the sun, as if receiving a blessing from above.

Now Labour is back in Brighton, led by a man the Guardian's Steve Bell once depicted – on his first day in No 10 – as a rain cloud. If nature had a feel for the political mood, she'd surely have arranged for slate-grey skies and unceasing rain. Instead, Labour's longest-ever spell in government is apparently ending as it started – with a get-together in the warm Brighton sunshine.

It's likely that next year's conference in Manchester will have the mood of a memorial service, a depleted reunion for the survivors of the blast. If so, they will look back on the Brighton-to-Brighton era of 1997 to 2009 as a single epoch: the age of New Labour. Like heirs to the deceased, they will rummage through their inheritance, wondering what to keep – and what to dump.

Gordon Brown offered his own audit of sorts yesterday, exhuming from the New Labour past an idea he had buried as soon as he entered Downing Street. The Brownites always loathed Blair's "respect agenda", regarding anti-social behaviour orders as dismal and sacking Blair's respect tsar. But Brown devoted a full page and a half of today's text to the topic, more than on foreign policy, defence and climate change combined.

So there were crowd-pleasing promises to crack down on Britain's "50,000 most chaotic families" and to set up "supervised homes" for teenage mothers. Shades of the Magdalene Sisters, but such talk has focus-grouped well, and, I'm told, Labour's polling suggests voters are sick and tired of doing their bit only to see others "play by different rules or no rules at all", as Brown put it.

In so doing, the prime minister duly revived what had been one of New Labour's defining ideas, captured in that slogan conceived by Brown and delivered by Blair: "Tough on crime, tough on the causes of crime." It paid dividends 15 years ago, and Brown is wagering it will work once more.

But Brown broke from New Labour's past too. For he accepted that antisocial behaviour exists at both ends of the spectrum, not confined to fly-tippers and hellish neighbours who menace the worst estates. He struck out at those at the top who also "break the rules", who also have responsibilities they are failing to meet – to wit, the bankers. Again the research was clear, says a Downing Street source: voters in the "squeezed middle feel ripped off by scroungers at the top and bottom". This was always the missing piece in the Blairite talk of rights and responsibilities. Too often it was the strong who had rights and the weak who had responsibilities.

Of course, rhetoric is the easy bit, and the government will have to demonstrate what genuine action it plans to take to rein in the bankers and their runaway bonuses. But for a party that once bragged of its comfort with the "filthy rich", to speak this way represented a sharp break from the recent past. It showed that the initial New Labour obsession with a big tent has gone: they now realise that some people belong on the outside, with those on undeserved mega-bonuses firmly in that category.

You could run a similar rule across the rest of Brown's address. He was New Labour-ish in his refusal to trumpet the 50p tax rate, still preferring stealthily-as-she-goes on matters of taxation, a topic that received only the most glancing treatment. On the other hand, the war on terror – a theme that dominated Blair's speeches in the early years of this decade – was barely mentioned.

Still, this can't be left to Brown alone. The wider centre-left needs to decide what should remain of the New Labour legacy. Sounding out delegates, former ministers, MPs and advisers today brought a range of responses so diverse that it suggests the party has not yet made up its mind about what worked in the New Labour experiment and what was a dreadful failure.

That said, there is close to a consensus on the debacle of foreign policy. Voices of left and right agree that Blair's doctrine of "liberal interventionism" is one part of the inheritance that should be dumped in the nearest skip. Even those who liked the idea in theory concede that its practice proved disastrous.

Equally momentous is the abandonment of a core part of New Labour thinking on the economy. "We gloried in a neoliberal economic policy when it gave us the boom," says one Brown aide. "We celebrated the freedom of people in the City to make grotesque sums of money when we believed it would pay for what we wanted to do. We now believe you have to intervene." This suggests more than a populist attack on bankers, but an ideological shift already under way.

Others yearn to see an end to triangulation, the split-the-difference politics that was such a Blair hallmark. Brown still lapses into that, offering, for example, not a full-blooded commitment to electoral reform but instead a modest alternative vote proposal on the ballot in the election after next. The same goes for his refusal to scrap ID cards outright.

There are plenty more items to be shoved on to the New Labour dustpile. "Fatuous slogans and pointless targets," offers Chris Mullin. "Initiative Tourette's syndrome," says Demos's Richard Reeves, noting Gordon Brown's inability to craft a big-picture, overarching argument without spoiling the effect by chucking in a few small-bore initiatives, just as Blair did before him.

But what should be kept? Strangely, Brown has abandoned the trait that once defined his public persona, especially as New Labour took shape. Brown used to be known for fiscal rectitude, carpeting any shadow cabinet colleagues who dared make an uncosted spending commitment. Yet yesterday, even as concern at the scale of public debt is rising, he was blithely showering money around, spending cash on a new National Care Service, for example, as if these were boom times. Aides insisted it would all be "revenue neutral", paid for by cuts elsewhere, but a more New Labour-ish Brown would have spelled that out at every paragraph.

There's more from the New Labour legacy that should be recovered or retained. The steps towards a more redistributive welfare state are taken for granted now, featuring only in the warm-up lists that precede speeches – the minimum wage, tax credits, winter fuel allowance, Sure Start centres, increases in child benefit and the rest – but they are a crucial part of the record, and classically Labour. The "New" bit was not to shout too loud about them, which looks now, amid sliding popularity, like a mistake.

Add to that devolution and a basic comfort with modernity – the New Labour era expanded gay rights and introduced civil partnerships – and you have a fair amount that is worth holding on to.

Above all, though, is that quality Peter Mandelson displayed so theatrically on Monday. The essence of New Labour was a sheer hunger for power, an absolute intolerance for the impotence of opposition. It is hard to maintain that appetite after 12 long years. And yet if Labour loses that, it will lose everything.